STEP to STL: export settings that keep curves smooth

Checked on 1 October 2026. The definitions below come from the Open CASCADE, FreeCAD, Autodesk Fusion and SOLIDWORKS documentation; the file sizes come from converting one sample part ourselves.

A STEP file stores exact curved surfaces; an STL stores only flat triangles. Converting one to the other means choosing how closely the triangles must follow the curves, and two numbers control that: the linear deflection (how far a facet may sit from the true surface, in millimetres) and the angular deflection (how sharply neighbouring facets may meet, in degrees). For FDM printing, 0.05 mm and about 11° keeps holes and fillets round at print scale without bloating the file. The STEP to STL converter with presets and custom tolerances uses exactly those two numbers, so you can see the effect before you download.

The two tolerances, in plain terms

The Open CASCADE meshing guide defines them this way: linear deflection limits the distance between a curve and its tessellation, and angular deflection limits the angle between subsequent segments. Every CAD program has the same pair under its own names. In FreeCAD they appear when you create a mesh from a shape, in Autodesk Fusion under the 3D Print refinement settings, in SOLIDWORKS among the STL export options, and in Open CASCADE, the geometry kernel the Omnvert converter uses, as the two meshing parameters:

Sources: the FreeCAD wiki page on Mesh from shape, Autodesk's 3D Print refinement settings and the SOLIDWORKS help page on STL export options.

What the numbers do on real holes

Which tolerance wins depends on the radius. Worked out from the geometry for a full circle, here is how many segments each preset asks for on a 5 mm hole, a 20 mm boss and a 100 mm cylinder (diameters in mm; Draft is 0.1 mm and 28.6°, Standard 0.05 mm and 11.5°, Fine 0.01 mm and 5.7°):

On small holes the angle decides: with 11.5° a 5 mm hole gets 32 segments even though 16 would already meet the 0.05 mm distance. On large cylinders the distance decides, because a fixed angle on a big radius leaves a wide gap. That is why setting only one of the two gives either faceted small holes or oversized large parts. The OCCT guide also notes that angular deflection can use a generic default, while linear deflection has an absolute meaning and should match your model.

File size against smoothness

We converted one sample part, a 120 × 80 × 57 mm block with 18 cylindrical and 9 spherical faces, at each preset. The volume was the same every time; the triangle count and file size were not.

A 0.4 mm nozzle laying 0.2 mm layers cannot show a 0.01 mm difference, so Fine and Ultra mostly cost slicing time and disk space on FDM. Resin printers with 0.05 mm layers and small pixels are where Fine starts to earn its size. Size also matters when you share: this part is 277 KB at Standard and 3.4 MB at Ultra, and on an assembly with dozens of parts that gap reaches tens of megabytes, enough to hit an upload limit; see STL upload limits for that case. Our tutorial on polygon count and print quality covers the same trade-off from the mesh side.

Which preset for which job

• Checking fit or size: Draft. Facets show on curves, but the file is small and opens instantly.
• Functional FDM parts, brackets, enclosures: Standard. Holes print round and the file stays in the hundreds of kilobytes for typical parts.
• Resin prints, small parts, fine threads: Fine. The extra triangles are what a 0.05 mm layer can actually show.
• Parts much larger than the bed, or reference geometry: Ultra or a custom value. On a one-metre part even a small angle leaves a visible gap, so the distance limit has to be tight.

How to convert STEP to STL with the right settings

1. Upload the CAD file. The converter reads STEP (.step, .stp), IGES (.iges, .igs) and BREP files up to 200 MB.
2. Pick a preset. Draft (0.1 mm, 28.6°) for a quick fit check, Standard (0.05 mm, 11.5°) for normal FDM prints, Fine (0.01 mm, 5.7°) for resin and small detailed parts, Ultra (0.002 mm, 2.9°) only for reference geometry or very large parts.
3. Or set your own numbers. Custom takes a deviation in millimetres and an angle in degrees. Tick Relative to part size if one file holds parts of very different sizes; the deviation then scales with each part.
4. Decide what to do with assemblies. By default you get one STL. Tick Also one STL per part to receive a ZIP with every part separately, and Also GLB if you want part names and colours kept for viewing.
5. Convert and read the result. It shows the file size, triangle count, model size in mm, volume, and the unit the STEP file was written in.
6. Check the size in your slicer. If the dimensions match your CAD model, you are done.

To try your own part, convert the STEP file to STL here, start at Standard and step up only if a curve looks faceted in the preview of your slicer.

Units: millimetres, inches and the 25.4 problem

STEP files record their unit; STL files do not. Slicers assume millimetres. If a CAD program exports an STL in inches, the part arrives 25.4 times too small. The Omnvert converter reads the unit from the STEP file and always writes millimetres, and the result line shows the source unit so you can confirm it. If you export STL from your CAD program directly, set the export unit to millimetres. The tutorial on scaling in STL shows how to spot a wrong unit.

Assemblies

An assembly exported as a single STL becomes several shells in one file. Where parts touch or overlap, the shells intersect, and slicers handle that differently: Cura merges overlapping volumes by default, others warn. For printing, one STL per part is cleaner, because each part can get its own orientation and settings. For sharing an assembly as one file with its parts intact, 3MF is the better container; see STL vs 3MF vs OBJ.

Limits

A finer tolerance cannot recover detail the CAD model never had, and it cannot fix a broken model; if some faces cannot be meshed even after healing, the converter says how many. Each preset also has a ceiling on triangle count, so Ultra on a very large assembly can stop with an error rather than produce a file nobody can slice. If you end up with an STL that is too heavy anyway, the STL reducer brings it down to a target size, and our article on reducing STL file size explains when re-exporting beats decimating.